1. Field of the Invention
The invention relates to an inductive rotary joint for transmitting electrical power between two units that are rotatable relative to each other, in particular for use in computer tomographs.
2. Description of the Related Art
Non-contacting inductive rotary joints are an advantageous substitute for the known mechanical slip-rings for transmission of electrical energy. In inductive transmission technology a coupling between rotatable units is effected with magnetic fields without contact. This has an advantage over mechanical slip-rings in that torque, wear, and thus, also an outlay of servicing are minimized. Furthermore, the surroundings of the rotary joints are not polluted by carbon dust.
Inductive rotary joints have at least one winding on each of the rotatable units. Furthermore, an iron core or ferrite core for controlling the magnetic field may be provided on the rotor, on the stator, or also on both parts. An alternating current signal is fed into a winding of one of the parts and tapped-off from another winding on the other part, and is supplied to a load. A rotary joint of this kind is disclosed, for example, in German Patent No. 29580172.
With a conductively coupled slip-ring it is possible, for example, for a constant voltage from a voltage source to be fed into the slip-ring and to be tapped off from a load on the other side in a simple manner. Because of the conductive connection of low ohmic resistance through the slip-ring, the output voltage will correspond to the input voltage, except for minor deviations. Owing to the low resistance of the slip-ring, a slight and usually negligible voltage drop depending on the load current is obtained.
With inductively coupled rotary joints, an equivalent circuit diagram of the transmission device will include a stray inductance as a series inductance between the input side and the load side. This stray inductance depends on the intrinsic inductance of the joint and, in particular, on the coupling factor. Especially with inductive rotary joints of large dimensions, it is often possible to obtain only a small coupling factor which, in addition, frequently fluctuates with the positions of the rotatable units relative to each other. Thus, for example, the coupling factor decreases with increase of an air-gap between the iron cores that are rotatable relative to each other. The stray inductance then increases accordingly. Now, in order to transmit higher power via the rotary joint despite this stray inductance, the stray inductance is used in suitable circuits like a discrete inductance. Its use would be, for example, as a storage inductance, or also as a resonance inductance. In the case of a resonance inductance, the inductance can be supplemented, for example, with a series capacitance to form a series resonance circuit, or with a parallel capacitance to form a parallel resonance circuit. Of course, more complex filter structures also may be obtained.
A rotary transmission device having resonance circuits is disclosed, for example, in European Patent No. 0953225. With circuits of this kind, it is a problem that a measurement means is always needed on the output side of the rotary joint. In most known resonance circuits having a series inductance, at least one, and usually also several of the output parameters, such as current, voltage, or power vary with a change of the load impedance. This is not a problem in the case of a contacting slip-ring, as the load is preferably fed at a constant voltage, which can be transmitted via the slip-ring without difficulty and is substantially independent of load.
However, in the case of a typical non-contacting rotary joint having a series inductance, the output voltage, output current, and output power change with a change of the load impedance. Furthermore, the series inductance is changed owing to mechanical tolerances during the movement of the rotatable parts relative to each other. In order to achieve a uniform supply to the output side, and to prevent a destruction of the connected components, it is necessary to regulate at least one of the electrical characteristics on the output side. For low power, a separate regulator such as a voltage regulator that is constructed to be a series regulator, or also a switching controller may be used. For higher power, at least one sensor for one of these electrical parameters should be provided on the output side. This sensor determines the magnitude of the electrical parameter(s), and signals the magnitude to the alternating signal source on the input side. Now an electrical parameter such as, for example, the current, voltage or frequency on the input side can be regulated with a control amplifier so that a supply is ensured, for example, at constant voltage. A technology of this kind is used in conventional switching power supplies.
With rotary joints, however, there is the problem that information from a sensor must be transmitted from the output side to the input side, i.e., between two units that are rotatable relative to each other. This requires a further rotary joint operating in an opposite transmission direction from that of the inductive power transmitter. A solution to this problem is disclosed, for example, in German Patent No. DE 29580172 U1 in the form of a capacitive coupling element. However, often no mechanical construction space is available for a capacitive coupling element of this kind, or else a coupling element of this kind is needed for data transmission for other data such as measurement data, for example, and therefore cannot be used for regulating purposes.
The problems described above increase with increase of size of the rotary joint. Thus, with compact units having diameters of a few centimeters it is still possible to use precise bearings with tolerances below 0.1 mm. With this, it is possible to achieve, for example, a precise air-gap of 0.2 mm, and a fluctuation in a range of 0.2 mm to 0.3 mm. With large units having diameters larger than 1 meter, as used for example in computer tomographs, the tolerances are already in a range of a few millimeters, and are partly greater than 5 mm. Thus, in a case like this the air-gap would vary between 1 and 6 mm, depending on position and operating conditions. This leads to a substantially larger stray inductance, which fluctuates substantially more strongly.